Transient Voltage Suppressor diodes question

Not really.

I think the bridge rectifier / two diodes in series idea was the most practical (Phil Allisons' suggestion IIRC?).

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John Devereux

Thanks John. Although i do not understand clearly how two diodes is seriec can work. I already posted a question in the previous discussion.

Rgds

Efthimios

Diodes start conducting in the "forward" direction at around 0.6V, so two of them in series start conducting at around 1.2V.

(The exact voltage depends on the current you put through them, as well as the temperature and the diode).

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John Devereux

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They will start to conduct at 1.2 Volt but how they will stop the 100 Volt pulse flowing in the same direction?

Efthimios

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By diverting the current to ground.

(use fixed point font to view):

Input ------------------------ | | V | --- [Sensor] | | | | V | --- | | | 0V ---------------------------

If the voltage goes above about 1.2V, the diodes conduct and keep the voltage below about 1.5V. This assumes that the 100V source is not able to supply more current than the diodes can stand. You mentioned

20mA, which is quite low, so any normal diode would be OK, 1N4148 or 1N4001 perhaps.

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John Devereux

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Thanks John you saved my day.

I once worked at a place that made specialized HV supplies (like an SEM); they originallly used MOVs (Metal Oxide Varistors) and they were an endless source of problems - MOVs wear out, and they have to send the box back to get them replaced.

Then the Transzorb press releases came out, and we not only started using them, but we issued an EO to take out all of the varistors and throw them away, replacing them with Transzorbs.

How "stiff" is that 100V? Transzorbs are spec'd in joules, I think, so should drag down the 100V source if it's draggable, But if all it is is a spike, it should be just fine and dandy. :-)

Cheers! Rich

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I had some burn-in failures that were clearly due to electrical overstress, not the kind of thing you see in burn-in. Transorbs on the burn-in boards took care of the problem, even though the person in charge of burn-in was certain there were no spikes on his well conditioned supplies.

Regarding the OP question, the answer is yes, but I would spec the transorb a bit higher than 1V since they turn on quickly.

He wanted to limit the 1V supply to less than 1.5V IIRC. No transorb will do that. And the "spike" is ~0.2s long. More of a "surge", I'd say.

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John Devereux

Then cascade a ferrite bead, an intermediate transzorb, maybe 25-40V, another ferrite bead, and a string of two 1N914's. Maybe another ferrite bead after them.

Good Luck! Rich